1. Field of the Invention
The present invention relates to a device for cooling an electronic unit constructed by combining electronic parts such as arithmetic elements or functional elements and, more particularly, to a device for cooling an electronic unit by passing the cooling air through a passage of a sealed structure and by releasing the heat from the electronic unit to the cooling air.
2. Related Art
In recent years, the electronic device has been used in various fields, and the degree of integration and the operating frequency of electronic parts composing the electronic device have risen higher and higher. Therefore, the calorific density has risen so that the electronic parts have taken high temperatures easily. In order to avoid malfunctions resulting from the temperature rise, there has been needed a cooling device having a higher capacity.
The air-cooling method for releasing the heat to the atmosphere is the most common for cooling the electronic device or electronic parts. This air-cooling method has been exemplified in the prior art by the forced air-cooling, in which the heat is evolved from the electronic parts by using a cooling fan to establish a cooling wind, or by the so-called “natural cooling”, in which the heat is evolved from the electronic parts by a cooling wind caused by the natural convection.
When the electronic parts are to be air-cooled, the efficiency of this air-cooling can be raised if the electronic parts are exposed directly to the cooling air, because of a high heat transfer rate from the electronic parts to the cooling air. In order to prevent the electronic parts from being contaminated with dust or the like or the circuits from being shorted, however, the cooling air has to be cleaned in advance by filtering it when the electronic parts are contacted directly by the cooling air.
Generally in the prior art, on the other hand, there is mounted on the vehicle an electronic control unit (as will be called the “electronic unit”) for controlling the engine or the like. This electronic unit generates heat when activated, so that the hot electronic unit is naturally cooled or forcedly cooled with an air-cooling device.
In the prior art, the electronic unit has been arranged inside of the vehicle. In recent years, however, there has been adopted the construction in which the electronic unit is arranged in the engine room.
When the engine controlling electronic unit is arranged inside, for example, electric wires (or a wire harness) for connecting the engine and the electronic unit have to be led in the engine room so that they are elongated. At the same time, the electric wires have to pass through a hole formed in the body panel partitioning the vehicle compartment and the engine room so that the works are troublesome to cause a rise in the cost.
If the electronic unit is arranged in the engine room, on the other hand, the through hole need not be formed in the body panel, and it is possible to shorten the length of the wires and to reduce the weight of and the cost for the concomitant parts. At the step of mounting the engine on the vehicle, moreover, the works to lead the electric wires can be simplified, and the electric wires can be prevented from being damaged or broken. In addition, the leading works are simplified to improve the maintainability and serviceability advantageously.
However, the engine room is in a hot atmosphere because the heat generated by the run of the engine is dissipated into the air of the room. In order to suppress the transfer of the heat in the engine room to the electronic unit, therefore, it is essential to arrange the electronic unit in the engine room while being confined in a housing box. Moreover, this housing box for housing the electronic unit is required to have not only the heat insulation and the cooling property but also the dust-proof and the water-proof sufficiently.
Especially, the electronic unit is intrinsically composed of the electronic parts which premise the use at the room temperature. Therefore, the electronic unit is warranted for its action when the ambient temperature around the electronic unit is at 80° C. or lower and when the temperature of the electronic elements in the electronic unit is at 105° C. or lower. For this warranty, there is needed not only the insulating function of the housing box but also the cooling function to suppress the temperature rise of the electronic unit.
This device for air-cooling the electronic unit is disclosed in Japanese Patent Laid-Open No. 9-207691. The cooling device disclosed is constructed such that an electronic unit composed of electronic parts is so housed in a cooling box having a cooling air passage defined and formed as to adjoin the passage. The electronic device is cooled by passing the cooling air through the passage. According to this construction, the electronic unit composed of the electronic parts or electronic circuits is not contacted directly by the cooling air so that the cooling device can be properly used even in case the electronic unit of the electronic parts is placed under relatively severe environmental conditions.
However, the so-called “inner two or three faces” of the cooling box constructing the cooling air passage are left as heat receiving faces so that the air-cooling device can cool only one electronic unit simultaneously.
In case the housing box of the aforementioned construction is modified to house two or more electronic units, on the other hand, the passage construction for the cooling air is so complicated that it cannot retain a sufficient cooling ability. Therefore, the temperature in the housing box may rise to cause malfunctions of the electronic units.
In this case, more specifically, these two electronic units occupy the housing capacity of the inside of the housing box at a high ratio. Therefore, it is difficult to retain a sufficient cooling passage for the electronic units with the resultant disadvantage that the cooling passage is short of cooling the electronic units.
If the cooling passage is enlarged or intensified to retain the sufficient cooling ability, therefore, the housing box itself is large-sized to cause another disadvantage that the place for the housing box is restricted.
In the housing box having the aforementioned cooling structure, moreover, the passage for passing the cooling air is arranged around the electronic unit. Therefore, the space is arranged to enclose the electronic unit so that the housing box itself is large-sized to occupy a relatively large volume of the engine room. Especially in the passage of the cooling air, the side walls of the passage act as the outer walls of the housing box or are contacted by or positioned near the outer walls, so that the cooling air to pass through this passage is seriously influenced by the heat transferred from the outside to the outer walls of the housing box. For reducing this influence and retaining the sufficient cooling ability, therefore, it is necessary to increase the flow rate of the cooling air and to retain a large internal volume of the passage. This necessity causes a tendency to enlarge the passage.
When the engine room has little surplus space for the arrangement, therefore, the enlarged housing box interferes with other parts thereby to cause a disadvantage that the works to assemble the parts are difficult.
Depending on the arrangement of the housing box, moreover, the ventilation of the wind of the radiator fan or the running wind may drop so that the cooling efficiency of the engine is insufficient. Therefore, the engine performance may be restricted, and the ability of the engine may not be fulfilled.
In the structure disclosed in the above-specified Laid-Open, moreover, the cooling box is opened upward, and the electronic unit of the sealed structure is arranged in the cooling box. Therefore, that portion on the upper side of the electronic unit, at which the temperature easily rise, is located on the open side of the cooling box so that it fails to receive the cooling through the cooling box. Moreover, the cooling air is fed from an intermediate position of one side portion of the cooling box placed in an upright position and is discharged from an intermediate position of the other side portion. Therefore, a falling flow and a rising flow of the cooling air are established in the cooling box so that the flow of the cooling air is hardly fulfilled by the natural convection. As a result, there is still left a room for improvement in the cooling efficiency.
In the disclosed construction, moreover, the portion of the cooling box on the upper side of an inlet and an outlet for the cooling air is left as the so-called “dead zone”, through which no cooling air passes, so that the electronic unit of the sealed structure has its substantial heat radiation area narrowed. There is also left a room for improvement in the cooling ability or the cooling efficiency.